This invention relates to an apparatus used for microwave heated reaction systems for performing physical or chemical processes within a microwave radiation field, for example for digesting materials to be analyzed by AAS/AES, ICP or other qualitative or quantitative techniques.
In some instances when heating is required to invoke a chemical reaction, there is concern over the loss of volatile components of the reaction mixture. This is often the case when mixtures are heated in open vessels. In the past, techniques for closed vessel heating of materials have been developed. Such techniques reduce or eliminate the loss of volatile components during the heating process.
Heating reaction mixtures in closed vessels can produce high pressures within the vessels resulting from the increased partial pressures of the individual components of the reaction mixture at elevated temperatures. Safety in the modern laboratory requires careful attention to the design and fabrication of reaction vessels to prevent catastrophic mechanical failure accompanied by violent ejection of reaction materials and potential injury to laboratory personnel (see chapter on safety in Skip Kingston's book).
Properly designed microwave reactor vessels incorporate a pressure release mechanism to safely and predictably vent excessive pressures within a vessel, in the event that the internal pressure of the reactor exceeds the recommended operating pressure of the vessel, and before the internal pressure exceeds the design limits of the vessel. In U.S. Pat. No. 5,382,414, herein incorporated by reference, a vessel is disclosed having a cover held in place by an elastic "spring" for releasing vapor when excess pressure builds within the vessel and then resealing the vessel.
There are alternatives to the use of closed vessels for heating reaction mixtures while limiting the loss of volatile components of the mixture. One approach is the use of vapor condensers connected to opened vessels. Such vapor condensing apparatuses cool the escaping vapors from the open vessels, below the boiling point of the escaping components, so that the vapors are condensed into their liquid state and will flow back into the sample vessel.
Among the patents that disclose the use of condensing units in combination with microwave radiation to supply heat to a system, external condenser units have been reported. For example, U.S. Pat. No. 4,826,575 discloses an apparatus for the production of high purity water by microwave heating technology. The condenser is a conventional unit available from any number of major laboratory glassware supply companies and is cooled traditionally by passing water through the condenser unit.
In U.S. Pat. No. 4,488,935, Ruhe discloses an apparatus for the continuous feed distillation of a fluid in a vacuum using solar/microwave energy. The apparatus includes a microwave energy source and a solar energy collector. However, the condenser coil is located outside the limits of the microwave cavity. Since many microwave ovens are designed with magnetrons or waveguides situated at a top portion of the ovens, such that the microwave field is projected downward into the cavity; only particular microwave ovens can accommodate the use of vapor condensers that penetrate the cavity or extend through the top of the microwave oven.
In U.S. Pat. No. 5,366,595, Padgett et al. disclose a mobile apparatus for pyrolyzing carbonaceous material and a method related thereto. Microwave radiation heats the waste material inside a cylinder to rapidly convert it into solid and fluid products. The fluid products escape outwardly from the sample containing cylinder and are transferred to a heat exchange chamber where they are cooled and any evolved vapors are condensed into liquid form and are further cooled. As disclosed, this apparatus also features a cooling system external to the microwave cavity.
More recent applications which describe an apparatus for distilling liquids by heating them inside a microwave heating chamber are disclosed by Heierli (U.S. Pat. No. 5,338,409; EP 0592443; WO 9209351) and Armstrong (U.S. Pat. No. 5,711,857). The condenser unit for these apparatuses is located externally to the microwave cavity for receiving and cooling vapors emitted from the liquid when heated within the microwave radiation field.
As was discussed in U.S. Pat. No. 4,826,575, it is also critically important that laboratory personnel be protected from exposure to microwave radiation. In the October 1980 issue of National Safety News an article by Howard Basson of the Food and Drug Administration, entitled Radio-Frequency and Microwave Radiation, sets out the FDA requirements on page 60 as follows:
For about 10 years, the FDA microwave oven performance standard has been in effect. Because 100 percent testing programs are required of all oven manufacturers, very few cases have existed where individual ovens have generated leakage levels of more than the maximum permissible level. The five mW/cm.sup.2 maximum permissible level, measured two inches (five centimeters) from an oven's surface, is a value that must not be exceeded over the working lifetime of any oven.
It is an object of this invention to provide an apparatus to perform heating if processes with reflux condensation, recovery, and retention of evolved vapors within a microwave radiation field. Traditional water-cooled condensing systems are not usable within a microwave cavity since the coolant will absorb the microwave radiation and heat up. In general it is difficult to use many traditional liquid condenser coolants as they possess a significant microwave absorption cross-section and hence will absorb energy and become hot. Also, the sample vessel to which the condenser is attached is desired to be rotated within the microwave cavity, to ensure homogeneity of exposure to the microwave field, thus adding further difficulty to a liquid cooling system particularly when more than one sample vessel is used at the same time.